Abu Dawud Book 2, Hadith Number 1307.

Chapter : Dozing during prayer.

Narated By Anas : The Apostle of Allah (PBUH) entered the mosque (and saw that) a rope was tied between two pillars. He asked: What is this (rope for)? The people told him: This is (for) Hamnah b. Jahsh who prays (here). When she is tired, she reclines on it. The Apostle of Allah (PBUH) said: She should pray as much as she has strength. When she is tired, she should sit down.

The version of Ziyad has: He said: What is this? The people said: This is for Zainab who prays. When she becomes lazy, or is tired, she holds it. He said: Undo it. One of you should pray in good spirits. When he is lazy or tired, he should sit down.